DIAGNOSTIC DEVICE FOR NOx SENSOR

ABSTRACT

The ECU performs an on-board diagnosis for a NOx sensor. The diagnosis is carried out when an exhaust system of an engine is fully cooled after shut-down of the engine. The diagnosis is carried out after a temperature in the exhaust passage is lowered to an ambient temperature. In the diagnosis proceeding, the ECU activates a heater attached on the NOx sensor. The ECU detects a heating performance of the heater and determines whether the heating performance is sufficient or not. The NOx sensor is in a stable condition after the engine system is fully cooled. Therefore, it is possible to perform the diagnosis while eliminating or suppressing influences of operating conditions of the engine system and a residual heat in the exhaust system.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-308967 filed on Nov. 29, 2007, the contents of which are incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a diagnostic device for a NOx sensor. The NOx sensor detects NOx components, nitrogen oxides, in an exhaust gas from the internal combustion engine.

BACKGROUND OF THE INVENTION

The NOx sensor for detecting a concentration of NOx components in the exhaust gas needs a periodical diagnosis in order to maintain accuracy and to warn malfunctions in an early stage. In order to carry out a proper diagnosis, the diagnosis shall be carried out when detectable engine conditions satisfy certain diagnostic conditions. However, the diagnostic condition is barely satisfied during the internal combustion engine is running, since the engine conditions are frequently varied. For example, an exhaust gas temperature may be varied in response to a change of the other engine conditions, such as an amount of a load on the internal combustion engine. Therefore, the engine conditions relating to the diagnostic condition could be instable and could not satisfy the diagnostic condition for a sufficient time period continuously. As a result, it is difficult to ensure accuracy of the diagnosis for the NOx sensor during the internal combustion engine is running

In order to overcome such a difficulty, JP2006-105965A proposes and discloses a diagnostic device for NOx sensor that performs a diagnosis using an electric control unit at a residual time after shut-down of the internal combustion engine. A system of the internal combustion engine usually has the residual time for which the electric control unit is still functioning continuously even after the internal combustion engine has been deactivated. The diagnosis in the prior art document is carried out in the residual time, since the engine conditions, such as the exhaust gas temperature, are relatively stable in a post shut-down state compared to a pre shut-down state where the internal combustion engine is still running. Therefore, at the post shut-down state, the diagnostic condition could be satisfied in a stable manner.

Although, the diagnostic condition could be stably satisfied in the post shut-down state compared to the pre shut-down state, there are still remaining disturbances, such as a difference in the exhaust gas temperature just before shut-down, a difference in a residual heat remaining in an exhaust system after shut-down, and a difference in an ambient temperature. The disturbances may influence and lower accuracy of the diagnosis. In addition, it is also difficult to determine whether the residual heat has sufficient heat mass for completing the diagnosis for the NOx sensor.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the present invention to provide a diagnostic device for a NOx sensor that performs a diagnosis with less influence of engine conditions.

It is a further object of the present invention to provide a diagnostic device for a NOx sensor that performs a diagnosis while the diagnostic condition is stably satisfied.

It is a still further object of the present invention to provide a diagnostic device for a NOx sensor that performs a diagnosis with high accuracy.

In one of preferred embodiments of the present invention, a NOx sensor includes a NOx sensitive element sensitive to a concentration of NOx components in exhaust gas from an internal combustion engine and a heater for heating the NOx sensitive element to an active temperature. A diagnostic device for the NOx sensor comprises means for detecting a temperature of the NOx sensor, means for detecting an ambient temperature, and means for diagnosing a heating performance of the heater by supplying power to the heater when the internal combustion engine is shut-down, and a difference between the temperature of the NOx sensor and the ambient temperature is lower than or equal to a predetermined threshold temperature.

Further, the diagnosing means permits the diagnosis for the heating performance of the heater when the temperature of the NOx sensor is almost the same as the ambient temperature.

According to the above described preferred embodiments, the temperature of the NOx sensor is stable and barely changed. Therefore, it is possible to perform the diagnosis while eliminating or suppressing thermal influence from surrounding components, such as thermal influence from a residual heat in the exhaust system. It is possible to suppress influence from engine conditions, keep stable diagnostic conditions, and improve accuracy of the diagnosis.

In a preferred embodiment of the present invention, the diagnosing means diagnoses the heating performance of the heater based on a reaching temperature of the heater for a predetermined time of power supply. In another preferred embodiment of the present invention, the diagnosing means diagnoses the heating performance of the heater based on a reaching time period of power supply until the temperature of the heater reaches to a predetermined threshold temperature. According to the embodiments using the reaching temperature or the reaching time period, it is possible to perform the diagnosis with simple configuration.

In a preferred embodiment of the present invention, the diagnosing means includes means for correcting the reaching temperature in accordance with the ambient temperature. In another preferred embodiment of the present invention, the diagnosing means includes means for correcting the reaching time period in accordance with the ambient temperature. For example, the reaching temperature of the heater after a predetermined time of power supply is lowered as the ambient temperature is lowered. The correcting means eliminates or suppress such an influence from the ambient temperature.

In a preferred embodiment of the present invention, the NOx sensor temperature detecting means detects the temperature based on a period of time from shut-down of the internal combustion engine. The temperatures of engine components are decreased as time is elapsed from shut-down the engine. Therefore, the temperature of the exhaust system where the NOx sensor is disposed is lowered close to the ambient temperature, when a sufficient time is elapsed from shut-down. By monitoring the elapsed time period from shut-down, it is possible to estimate the temperature of the NOx sensor based on the elapsed time period from shut-down of the engine. The embodiment enables the diagnosing means to achieve the temperature of the NOx sensor without activating a sensor. It is possible to eliminate the sensor for detecting the temperature of the NOx sensor. It is possible to shorten a period of time for performing the diagnosis, and avoid increasing of a number of components and complexity of the configuration.

In a preferred embodiment of the present invention, the NOx sensor temperature detecting means includes a sensor directly or indirectly detects a temperature in an exhaust passage through which an exhaust gas flows, and the diagnosing means further includes means for determining whether the diagnosis for the heating performance of the heater is permitted or not based on a difference between the temperature in the exhaust passage detected by the sensor and the ambient temperature.

In a preferred embodiment of the present invention, the diagnosing means terminates the diagnosis for the heating performance of the heater when the ambient temperature is lower than or equal to a predetermined threshold temperature. For example, power consumption in the heater increases significantly at a very low ambient temperature, such as at sub-zero degrees of Celsius. By terminating the diagnosis in such a very low ambient temperature, it is possible to preserve the charge level of the battery above a certain level for restart the engine.

In a preferred embodiment of the present invention, the diagnosing means terminates the diagnosis for the heating performance of the heater when a charge level of a battery is lower than or equal to a predetermined threshold level. Power consumption in the heater is relatively high. Therefore, if the battery is already discharged heavily before initiating the diagnosis, the power supply to the heater for performing the diagnosis may make the battery over-discharged or damaged By terminating the diagnosis in such a low charge level, it is possible to preserve the charge level of the battery above a certain level for restart the engine and protect the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In which:

FIG. 1 is a block diagram showing an engine system including a diagnostic device for a NOx sensor according to a first embodiment of the invention;

FIG. 2 is a block diagram showing a diagnostic device for a NOx sensor according to the first embodiment of the invention;

FIG. 3 is a flowchart according to the first embodiment of the invention;

FIG. 4 is a time chart according to the first embodiment of the invention;

FIG. 5 is a flowchart according to a second embodiment of the invention; and

FIG. 6 is a graph showing a relationship between an ambient temperature and a time for completing a diagnosis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of the invention is described below with the drawings. The first embodiment is an engine system having an on-board diagnostic device for a NOx sensor. The engine system may be used as a power source for powered machines, such as a vehicle, a power generator and a heat supplying device.

Referring to FIG. 1, a vehicle has an engine system 10. The engine system 10 includes an internal combustion engine 11 (hereinafter referred to as an engine 11), an exhaust purifying device 12, and an ambient temperature sensor 13 The engine 11 may be a known internal combustion engine, such as a reciprocating engine, e.g., a gasoline engine, a spark ignition engine, a diesel engine, and a self-ignition engine, and a turbine engine, e.g., a gas turbine engine. The engine 11 is a gasoline engine. The engine 11 includes a main part 14, an intake system 15, an exhaust system 16 and an electric control unit 17. The main part 14 includes a reciprocal mechanism having at least one piston 19 that reciprocates in a cylinder 18. An injector 21 for fuel supply is disposed on the cylinder 18. The injector 21 injects fuel into a combustion chamber 22 defined by the cylinder 18 and the piston 19. The injector 21 is a component of a direct fuel supply system that injects fuel directly into the combustion chamber 22. The engine 11 may use other fuel supply system, such as a pre-mixture chamber fuel supply system or an air-intake injection system.

The intake system 15 includes an intake pipe 24 for defining an intake passage 23 for air. The intake pipe 24 is communicated with the main part 14 at one end and is open to the atmosphere at the other end. The intake pipe 24 has an air filter 25 at a portion close to the end open to the atmosphere. The air filter 25 removes foreign matters. The intake passage 23 provides an air passage for introducing air from atmosphere to the combustion chamber 22 The intake system further includes a throttle 26. The throttle 26 adjusts an amount of air flowing through the intake passage 23 by adjusting an opening degree of a valve between a closed position and a fully open position. At least one intake valve is disposed in a downstream end of the intake passage 23. The intake valve is not illustrated. The intake valve alternately opens and closes a communication between the intake passage 23 and the combustion chamber 22 to control an air flow from the intake passage 23 to the combustion chamber 22. The amount of intake air is controlled by the throttle 26. It is also possible to control the amount of intake air by adjusting the intake valve operation, such as at least one of an opening timing, a closing timing, and an opening degree. Such an intake air control may be provided by an electrically operated valve system or a variable valve timing and lift system.

The exhaust system 16 has an exhaust pipe 28 for defining an exhaust passage 27. The exhaust pipe 28 is communicated with the main part 14 at one end thereof and is open to the atmosphere at the other end. The exhaust pipe 28 has a silencer, not illustrated, on the other end. The exhaust passage 27 discharges exhaust gas from the combustion chamber 22 to the atmosphere. At least one exhaust valve is disposed on an upstream end of the exhaust passage 27. The exhaust valve is not illustrated. The exhaust valve alternately opens and closes a communication between the exhaust passage 27 and the combustion chamber 22 to control an exhaust gas flow from the combustion chamber 22 to the exhaust passage 27.

Referring to FIG. 2, the electric control unit 17 controls the engine system 10 including the engine 11, the exhaust purifying device 12 and the fuel supply system. The electric control unit 17 is referred to as an ECU 17. The ECU 17 is configured as a micro computer including a CPU 31, a ROM 32 and a RAM 33. The ECU 17 may be linked with other control units for the engine system 10 via a local area network (LAN) on the vehicle. For example, the ECU 17 may have a data link with an intake control ECU via an internal LAN. The ECU 17 provides a fuel control means for controlling the fuel supply system in response to detected sensor signals, such as an operating degree of a gas pedal by a driver. For example, the ECU 17 outputs a drive signal for injecting fuel to the fuel injector 21. The ECU 17 controls an injection period that corresponds to an amount of fuel supplied to the combustion chamber 22 by adjusting a length of the drive signal.

Referring to FIG. 1, the exhaust purifying device 12 includes a three way catalytic converter 34, a temperature sensor 35 and a NOx sensor 36. The three way catalytic converter 34 is referred to as a converter 34. The converter 34 is activated when it heated above a certain active temperature. The converter 34 carries out an oxidation of unburnt hydrocarbons (HC) to carbon dioxide (CO2) and water (H2O). The converter 34 also carries out reduction of nitrogen oxides (NOx) to nitrogen (N2) and oxygen (O2).

The temperature sensor 35 is disposed in the exhaust pipe 28. The temperature sensor 35 includes a thermo sensitive element, such as a thermistor. The temperature sensor 35 outputs signal indicative of a temperature in the exhaust passage 27. The ECU 17 detects the temperature in the exhaust passage 27 based on the output signal from the temperature sensor 35. The temperature sensor 35 may be called as an exhaust temperature sensor 35.

The NOx sensor 36 is disposed on a position downstream to the converter 34 with respect to an exhaust gas flow in the exhaust system 16. The NOx sensor 36 detects a concentration of NOx components in the exhaust gas flowing through the exhaust passage 27. Referring to FIG. 2, the NOx sensor 36 has a NOx sensitive element 41 and a heater 42. The NOx sensitive element 41 includes a pair of electrodes not illustrated and a solid electrolyte disposed between the electrodes. The solid electrolyte is sensitive to the NOx components. The heater 42 heats the NOx elements 41 for maintaining a temperature of the NOx element above a certain active temperature.

The ambient temperature sensor 13 detects an ambient temperature outside the engine system 10. For example, the engine system 10 is mounted on a vehicle, the ambient temperature sensor 13 detects an ambient air temperature outside the vehicle. The ambient temperature sensor 13 outputs an electric signal indicative of the ambient temperature. The ECU 17 detects the ambient temperature based on the signal from the ambient temperature sensor 13. Therefore, the ambient temperature sensor 13 and the ECU 17 provide means for detecting an ambient temperature. The ambient temperature sensor 13 may be shared and commonly used with an air conditioning system for the vehicle.

Referring to FIG. 2, a diagnostic device 50 includes the ECU 17, the ambient temperature sensor 13, the exhaust temperature sensor 35 and NOx sensor 36. The ECU 17 is configured as a micro computer that has the CPU 31, the ROM 32 and the PAM 33. The ECU 17 provides functional blocks 51-56 by hardware components and software components, The ECU 17 provides a NOx input block 51, a power supply block 52, a heater temperature input block 53, an ambient temperature input block 54, an exhaust temperature input block 55, and a diagnosis block 56.

The NOx input block 51 is operatively connected with the sensor element 41. The signal from the sensor element 41 is supplied to the NOx input block 51. The ECU 17 detects a concentration of NOx components in the exhaust gas flowing through the exhaust passage 27 based on the signal fed to the NOx input block 51.

The power supply block 52 is operatively connected with the heater 42 in the NOx sensor 36. The power supply block 52 controls an amount of power supplied to the heater 42. The power supply block 52 may includes a power transistor for turning on and turning off the power supply to the heater 42. As a result, the power supply to the heater 42 is adjusted in an ON-OFF fashion.

The heater temperature input block 53 inputs a temperature Th of the heater 42 by calculating the temperature Th based on a voltage and a current supplied to the heater 42 from the power supply block 52 and a predetermined mathematical function. The heater temperature input block 53 detects the voltage and the current supplied to the heater 42 from the power supply block 52, and estimates the temperature Th of the heater 42 based on the predetermined mathematical function that can be obtained experimentally.

Alternately, the NOx sensor 36 may include a temperature sensor attached on the heater 42. The heater temperature input block 53 may input a signal from the temperature sensor attached on the heater 42 and detects the temperature of the heater 42. The temperature sensor may be a resistor element attached close to the heater 42. The heater temperature input block 53 may input a resistance observed on a resistor element.

The ambient temperature input block 54 is operatively connected with the ambient temperature sensor 13. The signal from the ambient temperature sensor 13 is supplied to the ambient temperature input block 54. The ECU 17 detects the ambient temperature based on the signal fed to the ambient temperature input block 54.

The exhaust temperature input block 55 is operatively connected with the exhaust temperature sensor 35. The signal indicative of the temperature of the exhaust passage 27 is supplied to the exhaust temperature input block 55. The ECU 17 detects the temperature in the exhaust passage 27 based on the signal supplied to the exhaust temperature input block 55. The exhaust temperature sensor 35 may be directly disposed in the exhaust passage 27 to directly detects the temperature in the exhaust passage 27. Alternatively, the exhaust temperature sensor 35 may be attached on the exhaust pipe 28. In this case, the temperature in the exhaust passage 27 can be indicated and detected indirectly.

The NOx sensor 36 is disposed in the exhaust passage 27. Therefore, the temperature of the exhaust passage 27 detected by the exhaust temperature sensor 35 and the ECU 17 roughly coincides with the temperature of the NOx sensor 36. The exhaust temperature sensor 35 and the ECU 17 provide means for detecting the temperature of the NOx sensor.

The diagnosis block 56 performs diagnosis for the NOx sensor by a program stored in the ROM 32. The diagnosis block 56 provides means for diagnosing the NOx sensor 36.

The ECU 17 includes a main relay 17 a. The main relay 17 a cyclically activates itself to activate the ECU 17 after the engine system 10 is shut-down by turning off a power switch such as an ignition switch 17 b. The main relay 17 a intermittently activates the ECU 17 to perform the diagnosis for the NOx sensor 36 by executing a program. The main relay 17 a alternates an OFF state and an ON state intermittently for a predetermined times or predetermined time period after the ignition switch is turned off. The main relay 17 a may be called as a self activate intermittent relay. The ECU 17 executes the program for the diagnosis in response to the turning on of the main relay 17 a. The main relay maintains the ON state for a certain short time period that is enough to activate the blocks 51-56 and perform the diagnosis. The interval for activating the main relay may be set within a range from a few minutes to a few tens of minutes. The interval may be adjusted to achieve a desired accuracy of the diagnosis and power consumption. The main relay 17 b provides means for activating the ECU 17 to perform the diagnosis for the NOx sensor 36 after the shut-down.

Referring to FIG. 4, the main relay maintains the ON state during the ignition switch is turned on. Then, the main relay is turned off to the OFF state, when a certain delay time is elapsed after the ignition switch is turned off. The main relay maintains the OFF state for a certain time period. Then, the main relay automatically wakes up and activates itself to the ON state even the ignition switch is turned off. In the ON state, the main relay activates the ECU 17. The main relay maintains the ON state for a certain short time period. After the ON state, the main relay again turns itself off to the OFF state. The main relay repeats above explained cyclic change between the ON state and the OFF state for a predetermined times.

Referring to FIG. 3, in a step S101, the diagnosis block 56 determines whether the engine system 10 is in a running condition or a shut-down condition after the program is started or resumed by turning on the main relay 17 a. The engine system 10 may be considered as the shut-down condition with high possibility during the ignition switch 17 b is turned off. However, in order to ensure the determination, the diagnosis block 56 inspects and determines that whether the engine system 10 is in the running condition or not, and whether the engine system 10 is in a starting condition such as a clanking condition in which a starter motor is activated or not. The diagnosis block 56 performs the step S101 based on an engine condition that can be monitored based on a detectable variable, such as at least one of an amount of fuel supplied by the injector 21 and a rotational speed of the engine 11 detected by a rotational speed sensor not illustrated.

In a case that the engine system 10 is in the shut-down condition, the diagnosis block 56 performs a step S102. In the step S102, the diagnosis block 56 determines whether the diagnosis is completed after the engine system 10 is shut-down or not. The diagnosis shall be performed at least one time during a period from shut-down to a restart of the engine system 10. But just one diagnosis is enough for the period. Therefore, the diagnosis block 56 terminates the flowchart, if one diagnosis is already completed during the time period from shut-down to the restart of the engine system 10.

In a case that the diagnosis has not yet completed after shut-down, the diagnosis block 56 performs a step S103, In the step S103, the diagnosis block 56 determines whether the diagnosis is permitted or not by determining whether the diagnosis condition is satisfied or not. In this embodiment, just one diagnosis condition is set. The diagnosis block 56 determines whether a difference between the ambient temperature and the temperature in the exhaust passage 27 is lower than or equal to a predetermined threshold temperature or not. As explained above, the temperature detected by the exhaust temperature sensor 35 may indicate the temperature in the NOx sensor 36, since the exhaust temperature sensor 35 is disposed in the exhaust passage 27. Therefore, the diagnosis block 56 performs the determination considering the temperature in the exhaust passage 27 detected by the exhaust temperature sensor 35 as the temperature of the NOx sensor 36.

The diagnosis block 56 may include means for correcting the temperature in the exhaust passage 27 to obtain more accurate value of the temperature of the NOx sensor 36. For example, the diagnosis block 56 may include means for estimating the temperature of the NOx sensor 36 based on the temperature in the exhaust passage 27 detected by the exhaust temperature sensor 35 by using a predetermined mathematical function. The diagnosis block 56 may apply a predetermined corrective coefficient to the temperature in the exhaust passage 27 detected by the exhaust temperature sensor 35 to simulate the temperature of the NOx sensor 36.

When the difference between the ambient temperature and the temperature in the exhaust passage 27 is lower than or equal to the predetermined threshold temperature, it is possible to consider the exhaust system 16 and the NOx sensor 36 are fully cooled and are in a thermally stable condition. The diagnosis block 56 determines whether the exhaust system 10 approaches enough to the ambient temperature and fully cooled or not. For determining the condition is satisfied, it is preferable that the predetermined threshold temperature is set zero (0) or a small value. The exhaust system 16 is surely fully cooled, if the difference between the ambient temperature and the temperature in the exhaust passage 27 reaches almost zero (0), in other words, the temperature in the exhaust passage 27 is almost equal to the ambient temperature. However, the predetermined threshold temperature may be set at a few degrees of Celsius or a few tens of degrees of Celsius, since unavoidable error on the temperatures detected by the sensors 13 and 35 should be taken into account. Therefore, the predetermined threshold temperature for determining fully cooled or not may be set in a range from zero (0) degree of Celsius to a few tens of degrees of Celsius in accordance with a required level of accuracy of the diagnosis for the NOx sensor 36 and preciseness of the ambient sensor 13 and the exhaust temperature sensor 35.

Alternatively, the diagnosis block 56 may determines whether the diagnostic condition is satisfied or not based on an elapsed time from shut-down the engine system 10. The exhaust system 16 is fully cooled and reaches the thermally stable condition, if a certain time is elapsed from shut-down of the engine system 10. Therefore, it is possible that the diagnosis block 56 determines whether the diagnostic condition is satisfied or not by determining whether a predetermined threshold time has been elapsed from shut-down of the engine system 10 or not. For example, the diagnosis block 56 may count the switching actions of the main relay 17 a or use a timer function embedded. Such a time based determination may be used instead of the temperature difference based determination described above. In addition, since the exhaust temperature sensor 35 is not used in such a time based determination, it is possible to determine the diagnostic condition without activating the exhaust temperature sensor 35. Therefore, it is possible to carry out the diagnosis for the NOx sensor 36 in a simple and faster manner.

In addition to the diagnostic condition determination described above, at least one additional condition may be used to the reaching temperature based determination or the reaching time based determination. For example, a condition of a battery or the ambient temperature may be used as an additional condition.

The diagnosis block 56 may determine that the diagnostic condition is not satisfied when a charge level of the battery, such as a voltage of the battery, is lowered below a certain level or the battery is damaged, e.g., an ageing. In this embodiment, the diagnosis for the NOx sensor 36 requires a power supply to the heater 42 that has considerably large power consumption. If the diagnosis for the NOx sensor 36 was carried out when the battery was discharged heavily or damaged, the battery could be over-discharged or further damaged, and might lose capability to activate a starter motor for next restart of the engine system 10. The diagnosis block 56 may terminates the diagnosis the charge level of the battery is lower than or equal to a predetermined threshold level. The above described battery based determination may ensure a restart of the engine system 10.

Further, the diagnosis block 56 may determine that the diagnostic condition is not satisfied when the ambient temperature detected by the ambient temperature sensor 13 it too low to preserve a sufficient charge level of the battery for the restart. The diagnosis block 56 determines whether the ambient temperature is lower than or equal to a predetermined threshold temperature or not. Then, the diagnosis block 56 terminates the routine for the diagnosis if the ambient temperature is lower than or equal to the predetermined threshold temperature. If the ambient temperature is lower than or equal to a predetermined threshold temperature that is a lower limit temperature, power consumption in the heater 42 could exceed excessive level and could makes the battery being over-discharged or damaged. In order to avoid such a trouble, it is possible to use an ambient temperature based determination in addition to the above determination for the diagnostic condition.

In the case that the diagnostic condition is satisfied in the step S103, the diagnosis block 56 initializes a counter C to zero (0) in a step S104. The diagnosis block 56 includes a counter embedded. Then, the diagnosis block 56 turns the heater 42 on in a step S105. In the step 5105, the diagnosis block 56 sets a diagnosis flag, and turns the heater 42 on in response to the diagnosis flag. The diagnosis flag is set during a diagnostic procedure is proceeding. The heater 42 is supplied with power via the power supply block 52.

Referring to FIG. 4, the main relay 17 a intermittently activates the ECU 17 to resume the diagnosis routine shown in FIG. 3. However, since the diagnostic condition is not satisfied until a time to, the diagnosis routine is terminated at the step 103 until the time t0. In an execution event of the diagnosis routine at the time t0, the diagnosis flag is set and the heater 42 is activated. Therefore, the whole of diagnosis routine first performed from the time t0. At the time t0, a certain time period has been elapsed from shut-down, and the engine system 10 is fully cooled down.

The diagnosis block 56 detects the temperature Th of the heater 42 in a step S106. The temperature Th increases as time elapsed. The temperature Th is obtained from the heater temperature input block 53. The temperature Th is also referred to as a reaching temperature at each event of the step S106. As explained above, the heater temperature input block 53 may input a signal from a temperature sensor attached on the heater 42 or input a resistance observed on a resistive element attached close to the heater 42.

In a step S107, the diagnosis block 56 determines that whether the temperature Th is higher than a predetermined threshold temperature T1 or not. The predetermined threshold temperature T1 is preferably set at the active temperature of the NOx sensor 36. The active temperature of the NOx sensor is usually a relatively high temperature such as 700 degrees of Celsius. The predetermined threshold temperature T1 may be set at another temperature not limited by the active temperature. For example, the predetermined threshold temperature may take a lower temperature than the active temperature. The predetermined threshold temperature shall be set within a range where the diagnosis can be performed with required reliability Such a lower threshold temperature may decrease power consumption of the heater 42 for the diagnosis.

In a step S108, the diagnosis block 56 increments the counter C when the temperature Th is not reached the predetermined threshold temperature T1. In a step S109, the diagnosis block 56 determines whether the counter C is greater than a predetermined threshold count C1 or not. In a case that the counter C is not greater than C1, the routine returns to the step 106 and repeat the steps S106-S109.

Referring to FIG. 4, the predetermined count C1 corresponds to a time period between the timing t0 and the timing t1. The predetermined count C1 indicates a predetermined heating time period of the heater 42. The heating time period is a time period for supplying power to the heater 52 via the power supply block 52.

In the case that the counter C is greater than the count C1, the diagnosis block 56 sets a heater flag in a step S110. The heater flag may be called as a heater malfunction flag, since a set state, an ON state, of the heater flag indicates that the heater 42 is malfunctioning. The predetermined count C1 is set as a maximum time period for increasing the temperature Th of the heater 42 up to the predetermined threshold temperature T1. Therefore, if the temperature Th does not reach to the predetermined threshold temperature T1 even after the count C1 is elapsed, this means that the heater 42 is not able to demonstrate expected heating performance due to some reasons. So, in such a case, it is possible to consider the heater 42 has some trouble, such as a malfunction, a damage or an aging. In this embodiment, if the heater 42 can not show required heating performance, the diagnosis block 56 determines that the heater 42 has a malfunction and set the heater flag.

In the case that the temperature Th exceeds the predetermined threshold temperature T1 before the predetermined count C1 is elapsed, the diagnosis block 56 determines that the heater 42 is functioning normal. Then, the diagnosis block 56 reset a heater flag to an OFF state in a step S111. The heater flag may be called as a heater normal flag, since a reset state, the OFF state, of the heater flag indicates that the heater 42 is functioning normally. While repeating the steps S106-S109, it is maintained that the counter C is smaller than or equal to the predetermined count C1 in the step S109. Therefore, if it is determined that the temperature Th exceeds the predetermined threshold temperature T1 in the step S107 while repeating the steps S106-S109, the counter C is smaller than or equal to the predetermined count C1. As a result, it is possible to determine that the heater 42 has no malfunction.

Alternatively, the heater flag may be divided into a malfunction flag and a normal flag. The malfunction flag indicates that the heater 42 is malfunctioning. The normal flag indicates that the heater 42 is functioning normal.

The diagnosis block 56 turns the heater 42 off in a step S112. Then, the diagnosis block 56 set a completion flag in a step S113. A set state, an ON state of the completion flag indicates that the whole processing for the diagnosis for the NOx sensor 36 is completed. The completion flag will be inspected in the step S102 to permit only one time of the diagnosis processing during one shut-down period from shut-down to a restart.

Referring to FIG. 4, the heater 42 is supplied with power from the timing toe The temperature Th of the heater 42 is increased until the counter C reaches the predetermined count C1. When the counter C reaches to the predetermined count C1 at the timing t1, the diagnosis flag is turned off and the power supply to the heater 42 simultaneously turned off. In the case of FIG. 4, the heater 42 would be determined normal, since the temperature Th reaches the predetermined threshold temperature T1 at almost the same time where the counter C reaches to the predetermined count C1.

The diagnosis flag and the completion flag are maintained during a shut-down state from shut-down to a restart, and may be reset at the restart of the engine system 10 or at next shut-down. The heater flag is maintained during the shut-down state and a following running state, since it shows a result of the diagnosis, which shall be reflected on a fuel supply control by the ECU 17 or shall be used to indicate a malfunction to the driver or a technician.

As explained above, the diagnosis for the NOx sensor 36 is carried out after a sufficient time is elapsed from shut-down, and the exhaust system 16 is fully cooled. Therefore, the temperature of the NOx sensor 36 and the temperature in the exhaust passage 27 are stable, since the temperature in the exhaust passage 27 is decreased as same as the ambient temperature. In such a condition, it is possible to heat the heater 42 and inspect a heater performance while eliminating influences from the operating condition of the engine system 10 and from the residual heat in the exhaust system 16. As a result, it is possible to perform the diagnosis with less influence of conditions of the engine system 10. It is possible to perform the diagnosis while the diagnostic condition is stably satisfied. It is possible to perform the diagnosis with high accuracy.

The count of the counter C1 indicates the time period until the temperature Th exceeds the predetermined threshold temperature T1. The time period for heating the heater 42 up to the predetermined threshold temperature T1 is monitored as a variable indicative of the heating performance of the heater 42, and is used to diagnose the heater 42. It is possible to diagnose the heater 42 by determining whether the heater 42 shows expected heating performance which is required to properly activate the NOx sensor 36. It is possible to perform the diagnosis for the NOx sensor 36 in a highly reliable manner with high accuracy. Further, in this embodiment, it is possible to perform the diagnosis without a sensor installed in the NOx sensor 36 for directly detecting the temperature Th of the heater 42. Therefore, high accuracy diagnosis can be carried out without increasing a number of components and increasing a complexity of configuration.

The diagnostic condition is determined based on the difference between the temperature in the exhaust passage 27 detected by the temperature sensor 35 and the ambient temperature detected by the ambient temperature sensor 13. It is possible to consider that the exhaust system 16 is fully cooled and the temperature of the NOx sensor 36 is stable, when the difference between the temperature in the exhaust passage 27 and the ambient temperature is smaller than or equal to a predetermined threshold temperature. Therefore, it is possible to eliminate or at least suppress an influence of the residual heat remaining in the exhaust system 16 from the diagnosis for the NOx sensor 36.

As explained above, it should be noted that the time based diagnosis condition determination may be used instead of the temperature based diagnosis condition determination. It is possible to eliminate an activation of the temperature sensor 35 for detecting the temperature in the exhaust passage 27. Therefore, in the case that the diagnosis block 56 employs the time based diagnosis condition determination in the step S103, it is possible to shorten the diagnosis and perform the diagnosis with simple configuration.

Second Embodiment

Referring to FIG. 5, a second embodiment of the invention is described below. In the following description, differences from the above described embodiments are mainly described. The same components and processing are described in the above described embodiment and can be understood by referring the above description. The second embodiment has the same blocks as shown in FIGS. 1 and 2. The second embodiment carries out a program as shown in FIG. 5.

The diagnosis block 56 resumes the routine in response to a self activation of the main relay 17 a. In a step S201, it is determined that whether the engine system 10 is running or not. In a step S202, the diagnosis block 56 determines whether the diagnosis is already completed or not. In a step S203, the diagnosis block 56 determines whether the diagnosis condition is satisfied or not. In a step S204, the diagnosis block 56 initializes the counter C to zero (0). In a step S204, the diagnosis block 56 turns the heater 42 on.

The diagnosis block 56 increments the counter C in a step S206, In a step S207, the diagnosis block 56 determines whether the counter C is greater than a predetermined threshold count C2 or not. Then, the diagnosis block 56 repeats the step S206-S207 until the counter C exceeds the predetermined threshold count C2. If the counter C exceeds the predetermined threshold counter C2, in step S208, the diagnosis block 56 detects the temperature Th of the heater 42. Here, the temperature Th may be detected by directly or indirectly as explained above.

In a step S209, the diagnosis block 56 determines whether the temperature Th, i.e., the reaching temperature, is greater than a predetermined threshold temperature T2 or not. If it is determined that the temperature Th is not yet reached to the predetermined threshold temperature T2, the diagnosis block 56 sets the heater flag to an ON state in a step S210. The heater flag may be referred to as a heater malfunction flag as well as the first embodiment. On the contrary, if it is determined that the temperature Th is reached already to the predetermined threshold temperature T2, the diagnosis block 56 resets the heater flag to an OFF state in a step S211.

The diagnosis block 56 measures a predetermined time period by repeating the step S206 and S207, and picks the temperature Th up at the timing when the predetermined time period is elapsed. Therefore, the diagnosis block 56 can determine whether the temperature Th of the heater 42 reaches to the predetermined threshold temperature T2 until the predetermined time period is elapsed from shut-down. In the case that the temperature Th reaches to the predetermined threshold temperature T2 in the step S209, it is possible to consider the heater 42 has sufficient heating performance and is functioning normal. On the contrary, in the case that the temperature Th does not reach to the predetermined threshold temperature T2 in the step S209, it is possible to consider the heater 42 has less and insufficient heating performance and may have a malfunction or damage. Then, in a step S212, the diagnosis block 56 turns the heater 42 off. In a step S213, the diagnosis block 56 sets the completion flag to the ON state.

In the embodiment, the diagnosis for the heating performance of the heater 42 is carried out based on a determination whether the temperature Th reaches the predetermined threshold temperature T2 or not until the predetermined time period is elapsed, i.e., the counter C reaches to the predetermined threshold count C2. It is possible to diagnose and inspect whether the heater 42 has the heating performance required to heat and activate the NOx sensor 36. It is possible to carry out the diagnosis for the NOx sensor 36 in a highly reliable manner with high accuracy.

The diagnosis for the heater 42 in the NOx sensor 36 can be carried out based on either the time period until the temperature Th reaches to the predetermined threshold temperature T1 as explained in the first embodiment, or the determination whether the temperature Th reaches to the predetermined threshold temperature T2 within the predetermined time period as explained in the second embodiment.

Other Embodiments

In the above described embodiments, several predetermined values are used. For instance, each of the predetermined threshold temperatures T1 and T2 is predetermined as a constant. Also, each of the predetermined threshold count C1 and C2 is predetermined as a constant. Alternatively, at least one of the predetermined values T1, T2, C1 and C2 may be obtained as a variable or a corrected value. For example, at least one of the predetermined values T1, T2, C1 and C2 may be varied in accordance with the ambient temperature detected by the ambient temperature sensor 13 in order to improve accuracy of the diagnosis and to reduce power consumption for the diagnosis. For example, at least one of the predetermined values T1, T2, C1 and C2 may be corrected based on the ambient temperature detected by the ambient temperature sensor 13.

Measuring the temperature increase of the heater 42 is influential to the ambient temperature. For example, a time period to increase the temperature of the heater 42 to the predetermined threshold temperature becomes longer as the ambient temperature becomes lower. Therefore, a typical time to complete the diagnosis for the NOx sensor 36 varies in accordance with the ambient temperature. Therefore, it is possible to improve the diagnosis by setting the predetermined values T1, T2, C1 and C2 as variables or correctable values. In order to achieve such an advantage, the above described embodiments may further include means for correcting the predetermined values T1, T2, C1 and C2. In the correcting means, the predetermined values T1, T2, C1 and C2 are corrected in accordance with the ambient temperature. It is possible to eliminate or suppress influences of the ambient temperature on the diagnosis. According to the modified embodiment, it is possible to improve accuracy for diagnosing the heating performance of the heater 42. The diagnosis block 56 may include means for correcting a reaching temperature or a reaching time period in accordance with the ambient temperature. The reaching temperature is the temperature Th of the heater 42 after the predetermined threshold time C1 is elapsed. The reaching time period is a period of time needed for increasing the temperature Th of the heater 42 up to the predetermined threshold temperature T2.

Further, in the embodiments above, the temperature Th of the heater 42, i.e., the temperature of the NOx sensor 36 is indirectly detected based on the voltage and current supplied to the heater 42. Alternatively, the NOx sensor 36 may include a temperature sensor to detect the temperature Th directly. Alternatively, the temperature of the NOx sensor 36 may be detected or estimated based on a period of time from shut-down of the internal combustion engine. The temperatures of engine components are decreased as time is elapsed from shut-down the engine. Therefore, the temperature of the exhaust system where the NOx sensor is disposed is lowered close to the ambient temperature, when a sufficient time is elapsed from shut-down. By monitoring the elapsed time period from shut-down, it is possible to estimate the temperature of the NOx sensor based on the elapsed time period from shut-down of the engine. This embodiment enables the diagnosis block to achieve the temperature of the NOx sensor without activating a sensor. It is possible to eliminate the sensor for detecting the temperature of the NOx sensor. This embodiment may shorten a period of time for performing the diagnosis, and avoid increasing of a number of components and complexity of the configuration.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims. 

1. A diagnostic device for a NOx sensor including a NOx sensitive element sensitive to a concentration of NOx components in exhaust gas from an internal combustion engine and a heater for heating the NOx sensitive element to an active temperature, comprising: means for detecting a temperature of the NOx sensor; means for detecting an ambient temperature; and means for diagnosing a heating performance of the heater by supplying power to the heater when the internal combustion engine is shut-down, and a difference between the temperature of the NOx sensor and the ambient temperature is lower than or equal to a predetermined threshold temperature.
 2. The diagnostic device for the NOx sensor claimed in claim 1, wherein the diagnosing means diagnoses the heating performance of the heater based on a reaching temperature of the heater for a predetermined time of power supply.
 3. The diagnostic device for the NOx sensor claimed in claim 2, wherein the diagnosing means includes means for correcting the reaching temperature in accordance with the ambient temperature.
 4. The diagnostic device for the NOx sensor claimed in claim 1, wherein the diagnosing means diagnoses the heating performance of the heater based on a reaching time period of power supply until the temperature of the heater reaches to a predetermined threshold temperature.
 5. The diagnostic device for the NOx sensor claimed in claim 4, wherein the diagnosing means includes means for correcting the reaching time period in accordance with the ambient temperature.
 6. The diagnostic device for the NOx sensor claimed in claim 1, wherein the NOx sensor temperature detecting means detects the temperature based on a period of time from shut-down of the internal combustion engine.
 7. The diagnostic device for the NOx sensor claimed in claim 1, wherein the NOx sensor temperature detecting means includes a sensor directly or indirectly detects a temperature in an exhaust passage through which an exhaust gas flows, and the diagnosing means further includes means for determining whether the diagnosis for the heating performance of the heater is permitted or not based on a difference between the temperature in the exhaust passage detected by the sensor and the ambient temperature.
 8. The diagnostic device for the NOx sensor claimed in claim 1, wherein the diagnosing means permits the diagnosis for the heating performance of the heater when the temperature of the NOx sensor is almost the same as the ambient temperature.
 9. The diagnostic device for the NOx sensor claimed in claim 1, wherein the diagnosing means terminates the diagnosis for the heating performance of the heater when the ambient temperature is lower than or equal to a predetermined threshold temperature.
 10. The diagnostic device for the NOx sensor claimed in claim 1, wherein the diagnosing means terminates the diagnosis for the heating performance of the heater when a charge level of a battery is lower than or equal to a predetermined threshold level. 